Stirling cryocooler

ABSTRACT

In a Stirling cryocooler, a displacer includes an internal space which is filled with gas. The displacer is reciprocatably accommodated in the expander main body. The displacer accommodates in the internal space one or more convection control plates, for controlling gas convection to a minimum. The one or more convection control plates are disposed along a line intersecting the displacer&#39;s longitudinal axis. At least one of the convection control plates divides the internal space into first and second sub-chambers, and includes an opening through which the sub-chambers communicate. The at least one of the convection control plates includes a convection control wall provided in the opening.

PRIORITY APPLICATION INCORPORATION BY REFERENCE

Priority is claimed to Japanese Patent Application No. 2014-062411, filed Mar. 25, 2014, and International Patent Application No. PCT/JP/2015/058178, filed Mar. 19, 2015, the entire content of which is incorporated herein by reference.

BACKGROUND

Technical Field

Certain embodiments of the present invention relate to cryocoolers, and particularly, to Stirling cryocoolers.

Description of Related Art

In the related art, in a Stirling cryocooler, a displacer may be shaped in a hollow form so as to accommodate a gas. In this case, it is well-known that heat loss arises due to heat transfer owing to convecting of gas inside the displacer. Accordingly, in order to control from gas convecting, technology whereby a cotton-like heat insulator, a strip-like heat insulator, or a powder-form heat insulator is accommodated in the displacer is known.

SUMMARY

One embodiment of the present invention affords a Stirling cryocooler comprising a displacer having a longitudinally extending gas-filled internal space, an expander main body reciprocatably accommodating the displacer, and at least one convection control plate for controlling gas convection to a minimum, accommodated in the displacer internal space, disposed along a line intersecting the longitudinal axis of the displacer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a Stirling cryocooler according to an embodiment of the present invention.

FIG. 2 is a view schematically showing an expander of the Stirling cryocooler according to the embodiment of the present invention.

FIGS. 3A and 3B are views schematically showing an example of an accommodation position of a convection control plate according to the embodiment.

FIG. 4 is a schematic view showing an internal configuration of a displacer according to another embodiment.

FIG. 5 is a view schematically showing an expander of a Stirling cryocooler according to another embodiment of the present invention.

FIG. 6 is a view schematically showing an expander of a Stirling cryocooler according to still another embodiment of the present invention.

FIG. 7 is a view schematically showing an expander of a Stirling cryocooler according to still another embodiment of the present invention.

DETAILED DESCRIPTION

It is desirable to provide a technology which more appropriately controls convection of gas inside a displacer.

In addition, arbitrary combinations of the above-described components, or components or expression of the present invention may be replaced by each other in methods, devices, systems, or the like, and these replacements are also included in aspects of the present invention.

According to the present invention, it is possible to provide a technology which more appropriately controls convection of gas inside a displacer.

During an operation of a Stirling cryocooler, a displacer reciprocates. Accordingly, in a case where a member which controls convection of gas inside the displacer is used, preferably, the member is fixed to the displacer. However, for example, if a cotton-like heat insulator, a strip-like heat insulator, or a powder-form heat insulator is used as the gas-convection control member, it is difficult to fix the member to the inner portion of the displacer. If the gas-convection control member cannot be fixed to the displacer, gas convection inside the displacer may not be appropriately controlled. In some cases, when the displacer reciprocates, the gas-convection control members collide with the inner wall of the displacer, or collide with each other, and thus noise may occur. Accordingly, a Stirling cryocooler according to an embodiment of the present invention uses a platelike member as the gas-convection control member.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In descriptions, the same reference numerals are assigned to the same elements, and overlapping descriptions thereof are omitted. In addition, configurations described below are exemplified, and do not limit the scope of the present invention.

FIG. 1 is a view schematically showing a Stirling cryocooler 10 according to an embodiment of the present invention. The Stirling cryocooler 10 includes a compressor 11, a connection pipe 12, and an expander 13.

The compressor 11 includes a compressor case 14. The compressor case 14 is a pressure container which is configured so as to airtightly hold a high-pressure working gas. For example, the working gas is helium gas. In addition, the compressor 11 includes a compressor unit which is accommodated in the compressor case 14. The compressor unit includes a compressor piston and a compressor cylinder, with one of either the compressor piston or the compressor cylinder being a movable member 15 that is configured so as to reciprocate in the compressor case 14, and the other of the two is a stationary member that is fixed to the compressor case 14. The compressor unit includes a drive source for driving the movable member 15 with respect to the compressor case 14 in a direction along a center axis of the movable member 15. The compressor 11 includes a support portion 16 which supports the movable member 15 to the compressor case 14 such that the movable member 15 can reciprocate. The movable member 15 vibrates with respect to the compressor case 14 and the stationary member at predetermined amplitude and frequency. The volume of the working gas inside the compressor 11 vibrates at specific amplitude and frequency.

A working gas chamber is formed between the compressor piston and the compressor cylinder. The working gas chamber is connected to one end of the connection pipe 12 through a communication path which is formed in the above-described stationary member and compressor case 14. The other end of the connection pipe 12 is connected to the working gas chamber of the expander 13. The working gas chamber of the compressor 11 is connected to the working gas chamber of the expander 13 by the connection pipe 12.

As described below with reference to FIG. 2, the expander 13 includes an expander main body 20, a displacer 22, and a support portion 40.

FIG. 2 is a view schematically showing the expander 13 according to the embodiment of the present invention. FIG. 2 shows an outline of an internal structure of the expander 13.

The expander 13 includes the expander main body 20 and the displacer 22. The expander main body 20 is a pressure container which is configured so as to airtightly hold a high-pressure working gas. The displacer 22 is a movable member which is configured so as to reciprocate in the expander main body 20. In addition, the expander 13 includes at least one support portion 40 which supports the displacer 22 to the expander main body 20 such that the displacer 22 can reciprocate.

The expander main body 20 includes a first compartment 24 and a second compartment 26. The first compartment 24 includes an expansion space 28 of a working gas which is formed between the expander main body 20 and the displacer 22. A cooling stage 29 for cooling an object is provided on the portion of the expander main body 20 adjacent to the expansion space 28. The second compartment 26 is configured so as to support the displacer 22 to the expander main body 20 via elastic members 30.

The second compartment 26 is adjacent to the first compartment 24 in a reciprocation direction (shown by arrow C in FIG. 2) of the displacer 22. A seal portion 25 is provided between the second compartment 26 and the first compartment 24, and thus, the second compartment 26 is separated from the first compartment 24. Accordingly, pressure variation of a working gas in the first compartment 24 is not transmitted to the second compartment 26, or a pressure of the working gas in the second compartment 26 is less influenced by the pressure variation of the working gas in the first compartment 24. In addition, gas which has the same kind as that of the working gas is enclosed in the second compartment 26 such that the pressure of the second compartment 26 is the same as an average pressure of the working gas supplied to the compressor 11.

The displacer 22 includes a displacer main body 32 which is accommodated in the first compartment 24, and a displacer rod 34. The displacer rod 34 is a shaft portion which is thinner than the displacer main body 32. The displacer 22 has a center axis which is parallel to the reciprocation direction of the displacer 22, and the displacer main body 32 and the displacer rod 34 are provided so as to be coaxial with the center axis of the displacer 22. The displacer 22 includes an internal space, and is filled with gas which has the same kind as that of the working gas. In addition, the displacer 22 includes one or more convection control plate 42 for controlling convection of gas. In addition, details of the displacer 22 will be described below.

The displacer rod 34 extends from the displacer main body 32 to the second compartment 26 through the seal portion 25. The displacer rod 34 is supported by the expander main body 20 in the second compartment 26 such that the displacer 22 can reciprocate. For example, the above-described seal portion 25 may be a rod seal which is formed between the displacer rod 34 and the expander main body 20.

The first compartment 24 forms a cylinder portion which surrounds the displacer main body 32. The expansion space 28 is formed between the bottom surface of the cylinder portion and the end surface of the displacer main body 32. The expansion space 28 is formed on a side opposite to a joint portion between the displacer main body 32 and the displacer rod 34 in the reciprocation direction of the displacer 22. A gas space 36 which is connected to the connection pipe 12 is formed between the joint portion and the seal portion 25.

A regenerator 38 is attached to the lateral surface of the cylinder portion of the expander main body 20 such as to be located along the outer periphery of the displacer main body 32. More specifically, the regenerator 38 is provided in the lateral surface of the cylinder portion of the expander main body 20 such as to be located in a cylindrical region having the longitudinal axis of the displacer 22 as its center axis, along the outer peripheral portion of the displacer main body 32. For example, the regenerator 38 has a laminated structure of metal meshes. A working gas between the expansion space 28 and the gas space 36 can flow through the regenerator 38.

A water-cooled heat exchanger 37 is provided between the regenerator 38 and the gas space 36. The water-cooled heat exchanger 37 performs heat exchange by which the working gas supplied to the compressor 11 is cooled and heat of the working gas is discharged to the outside of the expander 13. Moreover, a low-temperature heat exchanger 39 is attached to a portion between the regenerator 38 and the cooling stage 29.

The expander 13 supports the displacer 22 in the expander main body 20 at a plurality of different positions in the reciprocation direction of the displacer 22 such that the displacer 22 can reciprocate. Accordingly, the expander 13 includes two support portions 40. The two support portions 40 are provided in the second compartment 26. Therefore, it is possible to prevent the displacer 22 from being inclined to the center axis.

Each support portion 40 includes the above-described elastic member 30. The elastic member 30 is disposed between the displacer rod 34 and the expander main body 20 so as to apply an elastic restoring force to the displacer 22 when the displacer 22 is displaced from a neutral position. Accordingly, the displacer 22 reciprocates at a natural frequency which is determined by a spring constant of the elastic member 30, a spring constant due to the pressure of the working gas, and mass of the displacer 22. The displacer rod 34 is fixed to the elastic members 30 via elastic member attachment portions 51.

For example, the elastic member 30 is a spring mechanism which includes at least one leaf spring. The leaf spring is a spring which is referred to as a flexure spring, is flexible in the reciprocation direction of the displacer 22, and is rigid in the direction perpendicular to the reciprocation direction.

For example, the leaf spring is disclosed in Japanese Unexamined Patent Application Publication No. 2008-215440. The entire content of this related art document is incorporated herein by reference. Accordingly, the elastic member 30 allows the movement of the displacer 22 in the direction along the center axis, but prevents the movement thereof in the direction orthogonal to the direction along the center axis.

In this way, a vibration system including the displacer 22 and the elastic members 30 is configured. This vibration system is configured such that the displacer 22 is vibrated at the same frequency as that of the vibration of the movable member 15 of the compressor 11 so as to have a phase difference with respect to the vibration of the movable member 15. The displacer 22 is driven by pulsation of the working gas pressure generated by the vibration of the movable member 15 of the compressor 11.

A reverse Stirling cycle is formed between the expansion space 28 and the working gas chamber of the compressor 11 by the reciprocation of the displacer 22 and the movable member 15 of the compressor 11. In this way, the cooling stage adjacent to the expansion space 28 is cooled and the Stirling cryocooler 10 can cool the object.

Next, the displacer 22 will be described in more detail.

As described above, the displacer 22 according to the present embodiment is hollow, and has the internal space which is filled with gas which has the same kind as that of the working gas. Since the displacer 22 is hollow, weight of the displacer 22 decreases, and weight of the Stirling cryocooler 10 also decreases. In addition, the internal space of the displacer 22 is filled with the gas which has the same kind as that of the working gas, and thus, even when the gas inside the displacer 22 flows into the first compartment 24 or the second compartment 26 due to some reasons, it is possible to prevent the working gas from being contaminated.

Here, the internal space of the displacer 22 is in a vacuum state, and thus, it is possible to prevent the gas inside the displacer 22 from flowing into the first compartment 24 or the second compartment 26. In this case, if the internal space of the displacer 22 and the first compartment 24 or the second compartment 26 communicate with each other due to some reasons, the working gas flows into the internal space of the displacer 22, and thus, the working gas contributing cooling decreases. Accordingly, preferably, the internal space of the displacer 22 is filled with the gas which has the same kind as that of the working gas.

Cooling generated by the expansion space 28 is accumulated in the regenerator 38. Accordingly, the temperature of the end portion on the side of the regenerator 38 which is in thermal contact with the low-temperature heat exchanger 39 is lower than the end portion on the side of the regenerator 38 which is in thermal contact with the water-cooled heat exchanger 37. Hereinafter, in the present specification, the end portion on the side of the regenerator 38 which is in thermal contact with the low-temperature heat exchanger 39 is referred to as a “low-temperature end,” and the end portion on the side of the regenerator 38 which is in thermal contact with the water-cooled heat exchanger 37 is referred to as a “high-temperature end.” Similarly, the tip portion of the displacer 22 facing the expansion space 28 is referred to as a “low-temperature end,” and the base end portion facing the gas space 36 (that is, compression space) is referred to as a “high-temperature end.”

During an operation of the Stirling cryocooler 10, a temperature gradient in which a temperature decreases from the high-temperature end to the low-temperature end is generated in the regenerator 38. As shown in FIG. 2, the regenerator 38 is provided in the expander main body 20 so as to be positioned at a cylindrical predetermined region which has the longitudinal axis of the displacer 22 as a center axis in the outer peripheral portion of the displacer 22. In addition, the range in the reciprocation of the displacer 22, that is, a stroke length of the displacer 22 is shorter than that of the regenerator 38. Accordingly, in the displacer 22, a region which is in contact with the vicinity of the low-temperature end of the regenerator 38 and a region which is in contact with the vicinity of the high-temperature end of the regenerator 38 exist. For convenience of description, the position of the displacer 22 which is in thermal contact with the regenerator 38 may be referred to as a “position corresponding to the regenerator 38.” The “position corresponding to the regenerator 38” may be referred to as a “position of the displacer 22 facing the regenerator 38.”

As a result, in the gas which fills the internal space of the displacer 22, a temperature difference between the gas existing on the low-temperature end side of the displacer 22 and the gas existing on the high-temperature end side of the displacer 22 is generated. The reason why the temperature difference occurs is because the gas existing on the low-temperature end side of the displacer 22 is cooled by the low-temperature end of the regenerator 38 or the expansion space 28.

The expander 13 according to the embodiment may be installed such that the direction (the reciprocation direction of the displacer 22) in which the longitudinal axis of the displacer 22 extends is set to a horizontal direction, that is, a direction intersecting the gravity. In this case, in general, since weight of a low-temperature gas is heavier than weight of a high-temperature gas, convention is generated in which the low-temperature gas flows to the lower portion of the internal space of the displacer 22, and the high-temperature gas flows to the upper portion of the internal space. Accordingly, in the gas which fills the internal space of the displacer 22, a temperature gradient is generated in the direction intersecting the longitudinal axis of the displacer 22. More specifically, in the gas which fills the internal space of the displacer 22, the temperature of the gas existing on the lower side with respect to the gravity is lower than the temperature of the gas existing on the upper side.

Hereinafter, the expander 13 according to the embodiment has the assumption that the direction in which the longitudinal axis of the displacer 22 extends is the horizontal direction. Accordingly, the direction of the gravity is the direction intersecting the longitudinal axis of the displacer 22.

Since the temperature gradient is generated in the gravity direction in the gas which fills the internal space of the displacer 22, the temperature gradient is generated in the gravity direction in the regenerator 38 which surrounds the outer periphery of the displacer 22. That is, the temperature on the side of the regenerator 38 existing on the lower side with respect to the gravity is lower than the temperature on the side of the regenerator 38 existing on the upper side. The low-temperature end of the regenerator 38 is in thermal contact with the low-temperature heat exchanger 39 so as to perform heat exchange. However, according to a location, the temperature of the low-temperature end of the regenerator 38 is different from the temperature of the low-temperature heat exchanger 39, when the heat exchange is performed, heat loss is generated, and freezing performance of the Stirling cryocooler 10 decreases.

In the displacer 22 according to the embodiment, one or the plurality of convection control plates 42 which are disposed in the direction intersecting in the longitudinal axis of the displacer 22 are accommodated in the internal space of the displacer 22. Since the internal space of the displacer 22 is divided into a plurality of sub-chambers by the convection control plates 42, it is possible to control gas convection to a minimum over the entire internal space. Preferably, the convection control plates 42 are disposed along a line orthogonal to the longitudinal axis.

Preferably, the convection control plate 42 is a member has low emissivity and has high heat transmissivity. For example, the convection control plate 42 may be formed of an aluminum plate. Preferably, the convection control plate 42 is in airtight contact with the inner wall of the displacer 22. In addition, the regenerator 38 existing on the lower side with respect to the gravity and the regenerator 38 existing on the upper side are thermally connected to each other via the walls of the displacer 22 and the convection control plates 42. Accordingly, it is possible to decrease occurrence of the temperature gradient of the regenerator 38 in the gravity direction.

FIGS. 3A to 3B are views for explaining accommodation positions of convection control plates 42 according to the embodiment. More specifically, FIG. 3A is a view showing an aspect in which the displacer 22 is positioned at a bottom dead center, and FIG. 3B is a view showing an aspect in which the displacer 22 is positioned at a top dead center.

As shown in FIGS. 3A and 3B, a relative positional relationship between the regenerator 38 and the convection control plate 42 is changed according to the reciprocation of the displacer 22. As shown in FIGS. 3A and 3B, a convection control plate 42 a which is accommodated in the vicinity of the intermediate portion of the accommodation space of the displacer 22 exists at the “position corresponding to the regenerator 38” even when the displacer 22 reciprocates. Meanwhile, a convection control plate 42 b, which is accommodated in the side closer to the high-temperature end than the convection control plate 42 a in the accommodation space of the displacer 22, is deviated from the “position corresponding to the regenerator 38” according to the position of the displacer 22.

For example, as shown in FIG. 3B, when the displacer 22 is positioned at the top dead center, the convection control plate 42 b exists at the “position corresponding to the regenerator 38.” However, as shown in FIG. 3A, when the displacer 22 is positioned at the bottom dead center, the convection control plate 42 b is deviated from the “position corresponding to the regenerator 38” and is positioned at the “position corresponding to the water-cooled heat exchanger 37.”

As described above, since the regenerator 38 existing on the lower side with respect to the gravity is thermally connected to the regenerator 38 existing on the upper side by the convection control plates 42, it is possible to decrease occurrence of the temperature gradient of the regenerator 38 in the gravity direction. Accordingly, preferably, the convection control plate 42 is provided at the “position corresponding to the regenerator 38” even when the displacer 22 reciprocates. In FIGS. 3A and 3B, preferably, the convection control plate 42 is not accommodated in the position of the convection control plate 42 b, and is accommodated in the position of the convection control plate 42 a.

In this way, the convection control plates 42 are provided in the internal space of the displacer 22, and thus, it is possible to decrease the convection of the gas which fills the internal space, and it is possible to decrease occurrence of the temperature gradient of the gas in the gravity direction. As a result, it is possible to decrease occurrence of the temperature gradient of the regenerator 38 in the gravity direction. In addition, the regenerator 38 existing on the lower side and the regenerator 38 existing on the upper side in the gravity direction are thermally connected to each other via the connection prevention plates 42. Accordingly, this contributes a decrease of the temperature gradient of the regenerator 38 which is generated in the gravity direction. Therefore, a predetermined effect can be obtained if the displacer 22 has only one convection control plate 42. However, as shown in FIG. 2, if the plurality of convection control plates 42 are provided, it is possible to more effectively prevent occurrence of the temperature gradient of the regenerator 38 generated in the gravity direction.

Since the internal space of the displacer 22 is divided by one or the plurality of convection control plates 42, it is possible to decrease the convection of the gas which fills the internal space. However, even when the internal space of the displacer 22 is divided into a plurality of sub-chambers by the convection control plates 42, gas still exist in each sub-chamber. Accordingly, even when the internal space of the displacer 22 is divided by the convection control plates 42, it is difficult to completely prevent the heat loss generated due to the convection of the gas. In addition, the heat of the gas which fills the internal space is transmitted to the regenerator 38, the temperature of the regenerator 38 increase, and heat loss may occur.

Here, in addition to the convection control plate 42, a filling gas-convection control member may be accommodated in the internal space of the displacer 22.

FIG. 4 is a schematic view showing an internal configuration of the displacer 22 according to another embodiment. In the displacer 22 shown in FIG. 4, in addition to the convection control plates 42, filling members 43 for minimizing gas convection are accommodated in the internal space. Here, each of the filling members 43 is held in the internal space so as to be interposed between the convection control plates 42. Hereinafter, portions overlapping those of the internal configuration of the displacer 22 shown in FIG. 2 are described so as to be appropriately omitted or simplified.

As shown in FIG. 4, a portion between two convection control plates 42 different from each other is filled with the filling member 43, and thus, gas inside a sub-chamber divided by the convection control plates 42 decreases. The gas itself decreases, and thus, it is possible to prevent heat loss due to the gas convecting.

Here, a ratio of the filling member 43 occupying the internal space of the displacer 22 is greater than a ratio of the convection control plate 42 occupying the internal space of the displacer 22. Accordingly, in order to prevent an increase in the weight of the displacer 22, the filling member 43 is configured such that specific weight of the filling member 43 decreases. Specifically, the filling member 43 may be realized by a fibrous member or a net-like member. In addition, for example, since the inside of the displacer 22 may have a low temperature such as approximately 70K, the filling member 43 is configured of a material in which low temperature brittle fracture is not easily generated. Specifically, the filling member 43 can be realized using a synthetic resin polymer such as a fluorine-based resin or aramid resin, pumice stone (pumice), or the like. In order to prevent radiation, an aluminum tape may be bonded to, or an aluminum film may be sputter-deposited onto, the surface of the filling member 43.

In the displacer 22 shown in FIG. 4, the plurality of filling members 43 are spread so as to be in contact with the convection control plates 42 and the inner walls of the displacer, and thus, a laminated structure having multiple layers is obtained. In this way, the laminated structure is configured by the filling members 43, and thus, it is possible to decrease heat conduction from the filling members 43 to the regenerator 38.

As described above, one convection control plate 42 or each of the plurality of convection control plates 42 is a partition plate which extends along a plane intersecting the longitudinal axis direction C of the displacer 22, preferably, along a plane orthogonal to the longitudinal axis direction C. The partition plates divide the internal space of the displacer 22 into sub-chambers. Each of the sub-chambers may have air-tightness. The plate operates as a thermal bridge which is thermally cross-linked to the internal space of the displacer 22. The partition plate forms a thermal cross-link in the internal space of the displacer 22 from one side wall of the displacer 22 to other side wall thereof in the direction (for example, gravity direction) intersecting the longitudinal axis direction C. In addition, the gravity direction G is exemplified in FIG. 5.

The disposition of one or the plurality of convection control plates 42, particularly, the position of the convection control plate 42 in the longitudinal axis direction C of the displacer 22 and a gap between the convection control plates 42 may be variously set. For example, as shown in FIGS. 3A and 3B, one of two convection control plates 42 is positioned at an intermediate-temperature portion of the displacer 22, and the other thereof is positioned at a high-temperature portion of the displacer 22. In addition, as shown in FIGS. 2 and 4, the convection control plates 42 are arranged with equal gaps in the longitudinal axis direction C of the displacer 22. That is, the gaps between the convection control plates 42 have the same width as each other in the longitudinal axis direction C.

However, the convection control plates 42 may be disposed at locations different from those of the above-described embodiment. For example, the convection control plates 42 may be disposed with unequal gaps. As shown in FIG. 5, the convection control plates 42 may be densely arranged on the expansion space 28 side in the longitudinal axis direction C of the displacer 22. Accordingly, the convection control plates 42 may be sparsely arranged on the gas space 36 side. For example, the minimum gap between the convection control plates 42 in the longitudinal axis direction C is a half or less of the maximum gap therebetween.

At least three (five in FIG. 5) convection control plates 42 are accommodated in the internal space of the displacer 22. The convection control plates 42 includes a first plate which is positioned on the expansion space 28 side, a second plate which is positioned at the intermediate portion, and a third plate which is positioned on the gas space 36 side. The three partition plates are disposed so as to be adjacent to each other in the longitudinal axis direction C. A first sub-chamber is formed between the first plate and the second plate, and a second sub-chamber is formed between the second plate and the third plate.

A width W1 of the first sub-chamber in the longitudinal axis direction C is narrower than a width W2 of the second sub-chamber. A gap between the first plate and another plate (or tip portion of the displacer 22) adjacent to the expansion space 28 side may be the same as the width W1 of the first sub-chamber, or may be narrower than the width W1. A gap between the third plate and another plate (or base end portion of the displacer 22) adjacent to the gas space 36 side may be the same as the width W2 of the second sub-chamber, or may be wider than the width W2.

Alternatively, two convection control plates 42 may be accommodated in the internal space of the displacer 22. Contrary to the embodiment shown in FIGS. 3A and 3B, one of the two convection control plates 42 may be positioned at the intermediate-temperature portion of the displacer 22, and the other thereof may be positioned at the low-temperature portion of the displacer 22. In this case, a first sub-chamber is formed between the first convection control plate 42 on the expansion-space 28 side and the tip portion of the displacer 22, and a second sub-chamber is formed between the first and second convection control plates 42. The width of the first sub-chamber in the longitudinal axis direction C is narrower than the width of the second sub-chamber.

According to the embodiment shown in FIG. 5, it is possible to densely divide the internal space of the displacer 22 on the expansion-space 28 side in the longitudinal axis direction C. In this way, it is possible to more effectively control convection in the low-temperature portion. Accordingly, it is possible to prevent freezing performance from deteriorating due to convecting of the gas inside the displacer 22.

Particularly, since heat capacity is small in the low-temperature portion, the low-temperature portion is easily subjected to adverse effects due to the convection such as a temperature increase due to inflow of a high-temperature gas. In addition, the temperature gradient of a low-temperature portion of the regenerator 38 of the shown Stirling cryocooler 10 in the longitudinal axis direction C is steeper than the temperature gradient of a high-temperature portion thereof (a temperature difference per unit length of the low-temperature end in the axial direction is greater than a temperature difference per unit length of the high-temperature end). Accordingly, the convection is easily generated in the low-temperature portion. Since the convection control plates 42 are densely arranged on the low-temperature portion, and thus, it is possible to cope with the phenomenon in which the convection is easily generated in the low-temperature portion.

In general, convection is generated due to a posture of the expander 13 of the Stirling cryocooler installed at the site, particularly, a horizontal disposition (installation in which the longitudinal axis direction C is the horizontal direction) of the expander 13. According to the present embodiment, since deterioration in freezing performance due to convection is prevented, it is possible to install the Stirling cryocooler not only at a horizontal posture but also at an arbitrary posture.

In addition, similarly to the embodiment described with reference to FIGS. 2 to 4, in the embodiment shown in FIG. 5, the convection control plates 42 are positioned at the “positions corresponding to the regenerator 38.” During the reciprocation of the displacer 22, the convection control plates 42 are always positioned at a columnar region which is surrounded by the regenerator 38. Alternatively, at least one convection control plate 42 may be deviated from the “position corresponding to the regenerator 38.” At least one convection control plate 42 may be positioned outside the columnar region in at least a portion of the reciprocation of the displacer 22. For example, the convection control plate 42 which is disposed (that is, is positioned on the lowest temperature side) adjacent to the tip portion of the displacer 22 may be positioned at the “position corresponding to the low-temperature heat exchanger 39 or the cooling stage 29” when the displacer 22 is positioned at the bottom dead center.

The displacer 22 becomes heavy as the number of the convection control plates 42 increases. In a case where a decrease in weight of the displacer 22 is important, the convection control plate 42 may be provided on only the expansion space 28 side in the displacer 22. In this case, a relatively wide cavity is formed on the gas space 36 side in the displacer 22. The filling members 43 may be accommodated in this cavity.

According to an embodiment, each of the convection control plate 42 may have an opening 44. The opening 44 of the convection control plate 42 allows the first sub-chamber of the internal space of the displacer 22 adjacent to the first side of the convection control plate 42 to communicate with the second sub-chamber of the internal space of the displacer 22 adjacent to the second side of the convection control plate 42. The opening 44 is a degassing hole which is provided so as to easily evacuate the displacer 22. For example, when the expander 13 of the Stirling cryocooler is manufactured, the evacuation is performed so as to discharge air from the internal space of the displacer 22. After the evacuation is performed, the internal space of the displacer 22 is filled with the working gas of the Stirling cryocooler. In this way, the opening 44 may be provided on at least one convection control plate 42.

While the opening 44 is advantageous in manufacturing of the expander 13, the opening 44 may be a passage in convection of gas between the sub-chambers when the expander 13 is used. Particularly, this phenomenon significantly occurs in a case where the openings 44 in the convection control plates 42 are linearly arranged.

Accordingly, as shown in FIG. 6, at least one convection control plate 42 may include a convection control wall 46 that is provided on the opening 44. Each convection control plate 42 includes the opening 44 at the center thereof. The convection control wall 46 extends from the outer periphery of the opening 44 to both sides in the axial direction of the convection control plate 42. The convection control wall 46 may extend from the opening 44 to one side in the axial direction. In this way, side walls surrounding the opening 44 discontinuously extend along the center axis of the displacer 22. In a case where the opening 44 is circular, the convection control wall 46 is a cylinder. For example, an axial gap D between one convection control wall 46 and another convection control wall 46 adjacent to the one convection control wall 46 is within a range of ¼ to ¾ of the axial gap W between the convection control plates 42, and preferably, is ½ of the axial gap W. In this way, side walls are provided on the opening 44, and thus, it is possible to effectively control convection of gas between sub-chambers through the opening 44.

Instead of the convection control wall 46, as shown in FIG. 7, adjacent openings 44 may be displaced relative to each other. That is, the opening 44 in one convection control plate 42 and the opening 44 in another convection control plate 42 adjacent to the one convection control plate 42 may be positioned in locations different from each other in the plane intersecting the longitudinal axis of the displacer 22 (for example, the plane orthogonal to the longitudinal axis). Accordingly, it is possible to effectively control convection of gas between sub-chambers through the opening 44. Particularly, this off-set disposition of the opening portions 44 is effective for the case where the convection control plates 42 are densely arranged in the axial direction as the embodiment described with reference to FIG. 5. In addition, in some embodiments, a combination of the off-set disposition of the opening portions 44 and the convection control walls 46 may be used.

As described above, according to the Stirling cryocooler 10 of the present invention, it is possible to more appropriately control convection of gas in the displacer 22. Accordingly, it is possible to prevent deterioration in freezing performance due to convection of gas in the displacer 22.

It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention. 

What is claimed is:
 1. A Stirling cryocooler comprising: a displacer having a longitudinally extending gas-filled internal space; an expander main body reciprocatably accommodating the displacer; and at least one convection control plate for controlling gas convection to a minimum, accommodated in the displacer internal space, disposed along a line intersecting the displacer's longitudinal axis; wherein the at least one convection control plate divides the displacer internal space into first and second sub-chambers, includes an opening through which the first sub-chamber communicates with the second sub-chamber, and includes a convection control wall provided on the opening.
 2. The Stirling cryocooler according to claim 1, further comprising: a regenerator contained in the expander main body such as to be located outer peripherally along the displacer, in a predetermined cylindrical region having the longitudinal axis of the displacer as its center axis, wherein the at least one convection control plate is provided such as to be positioned, even when the displacer reciprocates, in a location corresponding to the predetermined cylindrical region in which the regenerator is positioned.
 3. The Stirling cryocooler according to claim 1, wherein the displacer accommodates a plurality of the convection control plates.
 4. The Stirling cryocooler according to claim 1, wherein the at least one convection control plate is in thermal contact with an inner wall of the displacer.
 5. The Stirling cryocooler according to claim 1, wherein: the displacer further includes a gas-convection minimizing filling member; and the at least one convection control plate retains the filling member.
 6. The Stirling cryocooler according to claim 5, wherein the filling member is composed of a synthetic resin polymer.
 7. The Stirling cryocooler according to claim 5, wherein the filling member is either fibrous or reticular.
 8. The Stirling cryocooler according to claim 1, wherein: the expander main body is configured to form an expansion space between the displacer and the expander main body, and to form a compression space between the displacer and the expander main body on a side thereof longitudinally opposite from the expansion space in the axial orientation of the displacer; and the displacer in its internal space accommodates a plurality of the convection control plates, densely arranged on the expansion-space side of the displacer and sparsely arranged on the compression-space side of the displacer along its longitudinal axis.
 9. The Stirling cryocooler according to claim 1, comprising at least two of the convection control plates, each having a sub-chamber communicating opening, wherein the sub-chamber communicating openings of adjacent convection control plates are displaced relative to each other in a plane intersecting the longitudinal axis of the displacer.
 10. The Stirling cryocooler according to claim 1, wherein the convection control wall extends outer peripherally from the opening in at least one sideward direction along the longitudinal axis of the displacer. 